In an induced-draft gas furnace, a gas valve typically establishes the flow of gas into a combustion chamber while a motor-controlled blower induces air and combustion gases through the combustion chamber. Variable draft-induced gas furnaces are generally of two types: multi-stage systems and modulating systems. In a typical multi-stage system, the blower motor has several fixed speeds and the gas valve has several fixed outlet pressures. When the user of a multi-stage system selects a thermostat setting, the system signals the gas valve to supply gas to the combustion chamber at a fixed rate corresponding to the selected thermostat setting. The system also signals the blower motor to induce a draft through the combustion chamber at a fixed rate corresponding to the gas flow rate.
A multi-stage system typically changes blower speeds based on input from one or more pressure switches. Such a switch can be triggered to switch on or off when pressure to or from the inducer blower exceeds or goes below a predetermined pressure value. However, other than indicating that a specific switch trigger pressure has been reached, a pressure switch does not provide the multi-stage system with information as to actual magnitudes of blower pressure on either side of the trigger value. Thus a multi-stage system can operate only at a few preset combinations of gas valve pressure and inducer blower speed. Operation may change from one to another of these combinations based on an imprecise gauge of blower pressure.
Modulating systems typically utilize variable-speed blower motors and electronically modulating gas valves. Modulating systems vary the gas valve outlet pressure by varying an electronic signal to the gas valve. Thus a modulating system can provide more precise control over gas flow than possible in a conventional multi-stage system. Another electronic signal that varies proportionately with the signal to the gas valve is used to vary the blower motor speed. Like multi-stage systems, modulating systems typically vary combustion levels based on trigger values for several pressure switches, but otherwise cannot sense inducer blower pressure levels. Thus, even though the speed of an inducer blower motor can be modulated, blower motor speed is varied imprecisely and indirectly. Such imprecise adjustments to air pressure and gas input to the combustion chamber do not always provide optimal air-to-gas ratios for combustion.